1. Field of the Invention
This invention relates to a process and apparatus for accurately locating and aligning dies in a die changing apparatus.
2. Description of the Prior Art
Because of the recent increase in variety of products which can be produced by a press work operation and the short life cycle of the products, a variety of products are produced in a small quantity in actual factories. When several sorts of the products are produced in the small quantity by the press work operation, it is required to change dies mounted on the power press every time when a different type of the press work is conducted. This increases the percentage of preparatory press work operations, such as, for example, the mounting or exchanging operation of the die, in the overall press work operation, which results in a decrease in the working efficiency of the power press.
In order to shorten and simplify the preparatory operations, various die changing apparatuses which are generally referred to as a quick change fixture, and makes it possible to mount, exchange or align the dies on the power press for a short period of time have been proposed, and some of the apparatus have been actually put to practical use.
One example of this type of the die changing apparatus is disclosed in the Japanese Patent Publication No. 46-7668 wherein the positioning of dies is performed in such a manner that a base plate on which a die is mounted is inserted into a guide hole formed in a vertically movable slider, the slider is actuated in the vertical direction by means of a pneumatic or hydraulic driving means so as to clamp the base plate onto a mounting surface or the base plate, and guide pins projected from the mounting surface of the base plate are inserted into holes on the base plate.
Applicant has proposed a die changing apparatus which is simple in structure and easy to handle as shown in FIG. 1. The die changing apparatus shown in FIG. 1 comprises an upper die fixture 2 mounted for vertically reciprocal movement and a lower die fixture 3. The upper die fixture 2 is guided in its reciprocable vertical movement by spaced guide bushings which receive guide posts 1 respectively, mounted uprightly in the lower die fixture 3. The upper and the lower die fixtures are provided with a pair of mounting devices 5 having stoppers 4 which are of substantially L-shape in cross section. A die 6 is secured to a base plate 7 in a precise manner in reference to its companion die and the base plate 7 is slidably received within two grooves with the two of its opposite edges thereby mounted one within each of a pair of mounting devices 5. The base plates 7 are accurately located within the fixtures by forcing four locating pins 8 retracted in the upper and the lower die fixtures 2 and 3 to enter companion openings 9 in the base plates 7 by means of pneumatic, hydraulic or rack and pinion mechanisms. When using the die changing apparatus shown in FIG. 1, replacement and mounting of the base plate 7 securing the dies can be achieved in a single step, which makes it possible to simplify the die changing operation and to shorten the time required for the die changing operation.
In the die changing apparatus shown in FIG. 1, the operations for accurately positioning and aligning dies 6 with respect to the base plates 7 are performed before and after or during operation of the power press as preparatory work. In order to achieve this die aligning operation accurately, it is usual to use a special die set which is independent of the die changing apparatus. This die set is generally referred to as a die setter and includes a nonreciprocating lower die fixture and an upper die fixture mounted for vertically reciprocal movement in the direction of the lower die fixture. When positioning and aligning the upper and the lower dies 6, the base plates 7 are secured to each of the upper and the lower die fixtures and the upper die fixture is lowered to determine fixing positions of the upper and the lower dies with respect to the base plates 7 so that appropriate clearance may be created between the upper and the lower dies, and then the upper and the lower dies are secured in position to the base plates.
The upper and the lower die fixtures of the die setter are provided with locating pins which are accurately in alignment with the locating pins 8 of the upper and the lower die fixtures 2 and 3 of the die changing apparatus shown in FIG. 1, and the positioning of the base plates 7 with respect to the die setter is effected by entering the locating pins into openings of the base plates 7. Accordingly, if each of the locating pins 8 of the die changing apparatus is accurately in alignment with each of the locating pins of the die setter, the base plates 7 after completing the die aligning operation by means of the die setter can be mounted on the die changing apparatus shown in FIG. 1 keeping the clearance between the upper and the lower dies as accurately as it is decided by the die setter. However, as a matter of fact, it is extremely difficult to effect machineworks which permit each of the locating pins 8 of the die changing apparatus to accurately align with each of the locating pins of the die setter, and errors are inevitably created when the die setter is subjected to the machinework. Accordingly, even if the die alignment in the die setter is perfect, there is a possibility of offset of the dies arising due to the errors inherent in the die setter when the base plates 7 are mounted on the die changing apparatus, and it is extremely difficult to restrict the offset of the dies less than several .mu.m. Furthermore, in the process for aligning the dies using the die setter explained hereinabove, it is necessary to use the die set in addition to the die changing apparatus, which increases die design cost and die manufacturing cost.